The Syrian military says Israeli jets have carried out an air strike on its territory, but denied reports that lorries carrying weapons bound for Lebanon were hit.
It said in a statement that the target was a military research centre northwest of the capital Damascus.
Two people were killed and five injured in the attack, it said.
Lebanese security sources, Western diplomats and Syrian rebels say an arms convoy was hit near Lebanon's border.
The attack came as Israel voiced fears that Syrian missiles and chemical weapons could fall into the hands of militants such as Lebanon's Hezbollah.
On Thursday, Russia's foreign ministry expressed "grave concerns" over the alleged Israeli attacks.
"If this information is confirmed, then we are dealing with unprovoked strikes at targets on the territory of a sovereign state, which grossly violates the UN Charter and is unacceptable, whatever motives are used to justify it," the statement said.
BBC Middle East correspondent Wyre Davies says none of the reports can be verified, although some well-placed diplomats and military sources say they would not be surprised if Israel had acted, given the recent instability in Syria.
Israel and the US have declined to comment on the incident.
The Lebanese military and internal security forces have not officially confirmed the reports, but say there has been increased activity by Israeli warplanes over Lebanon in the past week, and particularly in recent hours.
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